Maleic anhydride is used as a raw material for products ranging from agricultural chemicals, paints, paper sizing and food additives to synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed.
One important route to maleic anhydride involves the vapor phase oxidation of n-butane over a vanadiumphosphorus oxide catalyst. The reaction step involves oxidation of n-butane with air (oxygen) to form maleic anhydride, carbon oxides, water and smaller amounts of partially oxidized by-products. Typically, the process is carried out in fixed-bed reactors, fluid-bed reactors, or more recently in recirculating solids reactors having two reaction zones in which two separate reactions take place with a catalyst (the solid) circulating between the two reaction zones and taking part in reactions in both zones.
U.S. Pat. No. 4,192,951 ('951) discloses a vapor phase process for the oxidation of 4-carbon hydrocarbons to maleic acid and acetic acid with a molybdenum heteropolyacid catalyst. Disclosed catalysts include those in which the central phosphorus atom in heteropolyacids such as H.sub.3 PMo.sub.12 O.sub.40 is replaced with various transition metals, Si and Ge. The phosphorus atom is the heteroatom and occupies the tetrahedral site at the center of the heteropolyacid cluster. There is no disclosure of a heteropolyacid where the molybdenum is replaced by vanadium and a transition metal or main group cation.
The lattice oxygen of a vanadium phosphate catalyst is described in U.S. Pat. No. 4,668,801 wherein a vanadium phosphate catalyst is used for n-butane oxidation. Improvements such as an increase in selectivity are described for cases where a sub-stoichiometric amount of oxgen is used, or no oxygen is used over a vanadium phosphate catalyst in a recirculating solids reactor.
There are heteropolyacids described in the literature though most are characterized in solution, as opposed to the solid phase, or are characterized in the solid phase as the completely neutralized alkali salt. That is, all of the Bronsted acidity is neutralized in these salts which form as a precipitate. This makes the material essentially or nearly inactive for n-butane oxidation catalysis.
There is also mentioned in the literature the addition of secondary ions to preformed heteropolyacids. For instance, in Japanese Patent Application SHO JP59-36546, a catalyst system for n-butane oxidation is described containing H.sub.3 PMo.sub.12 O.sub.40 to which a secondary salt is added. For this catalyst system, copper and/or cesium and vanadium are placed on the outside of the heteropolyacid cluster, i.e., not on octahedral sites, but instead are part of a secondary structure where they neutralize some of the Bronsted acid sites, replacing H.sup.+ or H.sub.3 O.sup.+. These secondary metals are not part of the primary cluster structure.
In spite of the progress in catalyst and process development over the years, a need still remains to continue to improve heteropolyacids useful in the oxidation of n-butane to maleic anhydride, and it is to that end that the present invention is directed.